You need a WHERE clause - something like

update testing set fname = col1 + col2 where fname is null

otherwise it will just update the same 5000 records over and over for ever!

I found that...

Hi everyone ... thanks for your various ideas and suggestions.

I'm afraid that in the end I ran out of time and worked around the problem by changing the application logic!

Hi Steve,

Thanks for the suggestion.  I tried it out on my test database, which only has 1.8 million records on the relevant table, with the following results:

A straightforward

UPDATE table1 SET...